Abdominal and Gastrointestinal Radiology
Bowel gas pattern
Measure calibre: small bowel over 3 cm, colon over 6 cm, caecum over 9 cm. Valvulae conniventes cross the whole lumen; haustra are incomplete. Obstruction shows dilated loops proximal to a transition point with distal collapse. Closed-loop obstruction adds a U- or C-shaped loop, radially converging mesenteric vessels and a whirl sign, and is a surgical emergency. Strangulation is signalled by reduced wall enhancement, pneumatosis intestinalis, portomesenteric gas and free fluid, and demands urgent surgery rather than a trial of nasogastric decompression.
Free air
On supine films look for the Rigler sign (both sides of bowel wall visible), the falciform ligament sign and the football sign; on erect films look under the hemidiaphragms. Small volumes of gas are best detected on CT with lung windows.
Liver and biliary
Hepatocellular carcinoma in a cirrhotic liver can be diagnosed non-invasively: arterial phase hyperenhancement with portal venous washout and a capsule (LI-RADS 5). The common bile duct is dilated when it exceeds roughly 8 mm, allowing for age and prior cholecystectomy.
Pancreas and inflammation
Assess pancreatic necrosis on portal venous or delayed phase, not arterial. Fluid collections evolve over weeks and are named by their maturity.
Traps
Ileus is over-diagnosed as obstruction; always look for the transition point. Not every dilated loop means obstruction. Diverticulitis and appendicitis may both produce a sentinel loop, so inspect the colon and right iliac fossa specifically.
Rule of thumb
In a patient at risk (cirrhosis, chronic hepatitis B), a lesion meeting LI-RADS 5 criteria is treated as HCC without biopsy.